When I was in college, so many years ago. We learned that cement is strong
for compressive loads (such as columns in the cellar). Steel is good for
stretching loads (hanging a bridge from a frame).
If your friend's use of concrete is to support weight, it may work very
well. Concrete often has reinforcing bar, or rebar. This steel helps to
combine the compression strength of concrete with the stretching strength of
steel.

Fire protection of structural steel is placed on the exterior of the
members, not within a pipe or tube.
To make it simple, take a look at a typical pipe pile, driven empty, then
filled with concrete, with a circular cage reinforcing only in the upper
section (top 20-30'), a vertical bar or two near the center does nothing.
As Matt and Rico have stated the concrete fill is to resist buckling.
Tom

And I would submit that it does a lot more than just provide the
buckling resistance. Concrete is widely used to form structural
support to hold up buildings, bridges, etc. Even in this deck
example, what are the footers made of? They are essentially
cylindrical concrete pillars. So, while the concrete does keep the
steel pipes in a lolly column from buckling, which is clearly
important, the concrete also carries some of the weight directly. If
instead of concrete, you had some other means to keep the column from
buckling, say an internal criss cross web of little rods, I would say
the concrete filled lolly would carry substantially more weight.

First, I'm no engineer, but a few thoughts did come to mind...
You didn't mention the height of the deck or the span between support
posts, but in general there shouldn't be much of a load on a deck (no
walls, ceilings, roofing, etc.). Assuming a typical residential
situation, of course.
If the post footings are below the frostline in the area, they shouldn't
heave anyway should they? But even so, PVC is fairly flexible, and even
concrete flexes a small amount. With "typical" structural movements, I
don't see this being an issue. They're not gonna "snap like twigs" at the
first sign of stress.
If the PVC is filled with concrete, I don't see why it would be any
different than a cardboard sonotube, except you would not need to remove
the PVC after the pour. Even if the PVC doesn't offer any structural
support, the concrete piers inside the PVC should support the weight of
the deck.
If you're concerned about "flex" in the column, you should install rebar
reinforcement whether you're using PVC, a sonotube, or even building a
square box for the concrete pier.
Unless this is a second story deck, or one perched out over a hillside,
"most" decks only sit a foot or two off the ground and are supported by
multiple pier posts. I doubt there will be any significant sideways loads
to buckle the columns.
A recent article in Fine Homebuilding showed a system that used PVC pipe
to support a small shed (with no concrete). If it works for a shed, it's
bound to be adequate for a deck.
Unlike a steel post, PVC won't rust. However, regular PVC will be damaged
by ultraviolet light from the sun, and will get brittle. You could avoid
this by using grey PVC conduit which has protection against ultraviolet
light.
Around here, many decks are built with nothing more than a 4x4 post
sitting in precast concrete pier blocks sitting on the ground and have
lasted for decades. Also, many mobile homes sit on concrete blocks that
are just dry stacked piers with no reinforcement (ours was setup that way
for 13 years, with one end nearly four feet off the ground, and we
survived two earthquakes with no problems). These are obviously UNDER
engineered situations, but it's also possible to OVER engineer the
support for a deck as well...
Anthony

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